The invention pertains to the field of provision of low overhead ATM over SCDMA connectivity over a hybrid fiber coax plant between processes coupled to customer premises cable modems and head end cable modems using an optimized ATM cell with smaller header and using synchronous code division multiple access in at least the upstream direction.
In order to provide bidirectional digital data communication over a cable TV coaxial network to multiple subscribers with multiple services available over a single coax cable (hereafter called interactive systems), several problems have to be solved. First, there is the problem of noise and interference. A second major problem, but related to the first problem, is synchronization of data transmission so that effective, error-free communication can be achieved. Cable networks typically involve a so-called head end or central unit from which video is transmitted to subscribers coupled to one or more main trunk lines from which extend numerous branch lines which may enter subscriber homes or which may couple to other branch lines. At each junction of a branch line to the trunk line or another branch line there is a directional coupler which is intended to direct transmissions from the head end to the subscribers in one direction and to direct transmissions from the subscribers back to the head end without leaking energy intended for transmission to the head end into branch lines coupled to other subscribers. In order to send digital data over video coax, a modem is necessary at both the head end and at all the subscriber locations to modulate digital data onto the coax as RF signals, and to receive RF signals carrying digital data and derive the digital data therefrom. Because RF signals are propagating along the cables, and because the couplers are not perfect, reflections occur at the directional couplers that cause noise and interference. This is because the reflections are frequently of the opposite polarity depending upon the impedance mismatch and the distances involved. These reflections are therefore sometimes additive and sometimes subtractive, thereby resulting in random variations in the amplitudes of the RF signals. These random variations make discrimination during the demodulation process to derive the digital data more difficult.
Further, because the subscribers are at physically different distances from the head end, the signals from each subscriber's modem arrive at the head end at different times because of different propagation delays. Because digital data is transmitted in frames and because all subscribers must be synchronized to the same frame timing, these different propagation delays for each subscriber cause problems in synchronizing data.
In the typical interactive system, there are bidirectional amplifiers. Each amplifier has two channels, one of which amplifies signals in a high frequency range from 45-750 mHz for transmission of data from the head end to subscribers, and the other of which amplifies signals in a low frequency range from 5-42 mHz for transmission of data from the subscribers to the head end.
Interactive systems typically involve in excess of one hundred different channels on which separate digital data streams can flow in addition to the separate channels on which the video signals are provided for normal cable TV service. To send digital data as RF signals, very complex constellations of separate amplitude and phase combinations are used to encode the digital characters being transmitted. Because of the large number of data points, the differences in phase and amplitude between the different points are not large. Therefore, the impairments described above can cause errors by causing misinterpretation by demodulators of what characters were actually sent.
All of the above applies to the physical layer of the OSI model for data interchange between computers. At the higher protocol levels in the OSI model there are several standard protocols that are currently known. One of these protocols is the TCP/IP protocol used on the Internet. This protocol is not satisfactory for provision of high demand services such as video teleconferencing and video on demand since TCP/IP has no provision to guarantee quality of service and provide guaranteed bandwidth capacity. Because there is no concept of reservation of bandwidth in TCP/IP protocol, it is not suitable for delivery simultaneously of audio, video and data services to multiple subscribers.
The ATM protocol is the currently favored local area network protocol which is designed to simultaneously deliver integrated voice, video and data services. However, the ATM protocol was designed for local area networks where there is no shared media which is used to simultaneously deliver ATM cells between more than one pair of communicating devices. ATM is a point to point communication protocol that cannot be directly used on a CATV plant with its point to multipoint/multipoint to point topology.
A major problem addressed by the method disclosed herein is the high overhead and wasted bandwidth associated with standard 53 byte ATM cells. Since bandwidth is at a premium and a standard 53 byte ATM cell has a 5 byte header, not all of which is needed in the HFC environment described above, some bandwidth approaching 10% is wasted by use of standard ATM cells.
Therefore, a need has arisen for a more bandwidth efficient method of using ATM protocols to support interactive digital systems carried out over HFC cable plants.